When the prelude window appears, I quite often am so startled that I stop
working automatically (which is a good thing, I guess :-)

However, if I want to postpone my break (without hitting the postpone button), I
have to continue working. With the current settings, I must be active for one
second to be considered active, within 5 seconds after the prelude is presented.

I very often manage to be too slow. Then, I get a break window, and have to
manually click "postpone".

I propose to show the prelude for a longer period of time (say 10 seconds?),
even when the user is inactive. This will give me more opportunity to continue
working.

Agreed.